I have a sof that is covered with some type of coated vinyl. It is glued up so no seams. Sad news, it seems to have a leak. There are no obvious tears or holes.
I have had two suggestions: fill with water and see what happens and pressurize the hull with my compressor, coat the hull with soapy water and see where the bubbles happen. I suspect there will be a problem with one of the seams.

I've taken it out on two paddles and had water in the hull at the end of the trip maybe a gallon to three gallons. Rough estimate.
The boat is a baidarka, long and thin, nicely made but I think it will be leaving my fleet.
So I like to repair and offer it to someone who wants a missile on the water.